The present invention concerns a sound attenuator for low frequencies for air-conditioning ducts, in particular for intake-air and/or exhaust-air ducts in paper mills.
Ever stricter requirements are imposed on suppression of noise in the environment. One important source of noise consists of the intake and exhaust air pipes for ventilation in connection with various industrial plants and other large buildings, through which pipes especially the noise of blowers is spread into the environment. The blowers are usually chosen on the basis of the quantity of air produced by them, and attention is frequently not paid to the noise produced by them. The noise produced by the blowers has quite a wide spectrum, which also imposes particular requirements on the noise suppression.
In view of suppression of noise, paper mills are particularly demanding, because the ventilation of the paper machine hall and in particular the elimination of moisture from the drying section of the paper machine require large quantities of air. It is a particular problem in the air-conditioning of a paper machine, e.g. with respect to the exhaust air from the wire part, that the exhaust air contains a large amount of moisture and a certain extent of fibers and various paper fillers, such as kaolin clay. Said materials tend to block the sound attenuators.
The prior-art sound attenuators in general, and the sound attenuators used in paper machine ventilation in particular, have been difficult to maintain, for they are often difficult to clean, and the replacement of their attenuator members is often difficult. As a consequence, sound attenuators which have been designed an dimensioned efficiently in of themselves operate unsatisfactorily, because they are "blocked" as a result of impurities.
Since the noise produced by blowers has quite a wide spectrum, in the intake and exhaust air ducts connected to the blowers, it is frequently necessary to used both absorptive and reactive sound attenuators. Absorptive sound attenuators operate primarily at higher frequencies; the maximum of their attenuation is at a frequency of about 1000 Hz, whereas reactive sound attenuators, which are mainly based on various resonator constructions, operate most efficiently at low frequencies, and their maximum attenuation is, as a rule, tuned in a range of about 100 to 200 Hz.
For sound attenuation at low frequencies, there are various principles, whose applications have been used and are used in sound attenuators, as is well known.
One well-known application in sound attenuation at low frequencies is the plate resonator, i.e. membrane attenuator, wherein the oscillation of the plate converts the sound energy to thermal energy. In a plate resonator, the plate or membrane that operates as the mass is placed at a certain distance from a rigid wall and closes the air space, which operates as a spring, behind it. In a plate resonator, the maximal attenuation occurs at a resonance frequency which depends on the mass of the plate and on the distance between the rigid wall and the plate that operates as the mass.
A second well-known solution for sound attenuation at low frequencies is the Helmholtz resonator, in which the resonator consists of an air space which communicates with the "outer air" through an opening. An air plug present in the opening forms the mass that resonates on support of the spring force formed by the air enclosed in the hollow space. The resonance frequency of a Helmholtz resonator depends on the area of the opening, on the volume of the air space, and on the length of the air plug formed in the opening. In a Helmholtz resonator, the frequency range and the extent of attenuation can be regulated by changing the dimensions of the chamber that operates as the air space and the size of the opening. When the volume of the air space becomes larger, the resonance frequency is shifted toward lower frequencies. When the area of the opening is made smaller, the resonance frequency is shifted towards lower frequencies.
With respect to prior art related to the present invention, reference is made to U.S. Pat. No. 4,787,473, in which a solution is suggested for attenuator members in which there are Helmholtz resonators at both sides. In the version described in said U.S. patent, attention has, however, not been paid to a possibility of cleansing or to ease of maintenance of the attenuator units, nor to designing the attenuator so that it is suitable for different purposes of use.
One important property of sound attenuators connected to the input or output ducts of blowers is, besides the attenuation of noise, the loss of pressure produced by them.